Um.... I'm sure that myself and others would be very interested to know your reasoning behind "chipping" a 4 year old child. I would feel that this is definitely getting into issues of consent.
In many ways (as I see it) implantation of an RFID chip falls very well within the "informed consent" area of knowledge. We research the risks associated with the procedures and technology that we are using and make an informed personal decision.
I would say that a 4 year old child has not reached the level of maturity to make an informed decision on this. This is not a medically necessary procedure and serves no medically necessary purpose. (It barely even falls inside the category of medical procedure)
So, that being said, @usafvet19: Why do you want to implant RFID technology in her? Shouldn't this be her choice when she reaches an age where she can legally make an informed decision on the topic?
I would venture to say that you probably won't receive much support at all in this concept without at least saying why you want to do this. (Not to mention that I don't know of any reputable body modification shops that will do ANYTHING with a 4 year old girl.)